Preventing the formation of carbon



United States Patent'O PREVENTING THE FORMATION OF CARBON SLUDGE IN FUSED BATHS Giles F. Carter, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Application May 10, 1954, Serial No. 428,851

1 Claim. (Cl. 252156) This invention relates to the molten salt baths containing alkali metal hydrides which are commonly used for removing oxides from the surface of metallic articles. More particularly, the invention relates to covers for hydride baths to prevent oxidation of the hydrides by exposure to the air. Still more particularly, it relates to covers made from finely divided carbon in such a fashion as to avoid the formation of a carbon sludge in the bath.

Gilbert, in U. S. Patent 2,377,876, discloses a process for removing oxide from the surface of, or descaling, steel and other metals by treating the metals in a bath of molten alkali metal hydroxide containing about 1-20% by weight of alkali metal hydride dissolved therein. This process is particularly useful for descaling stainless steel and other ferrous materials.

In commercial practice steel articles are immersed in a fused bath of sodium hydroxide containing 15% by weight of sodium hydride, the bath generally being maintained at a temperature within the range 350450 C. The hydride is usually produced by reacting hydrogen or cracked ammonia with metallic sodium floating on a restricted area of the bath surface and then distributed throughout the bath. The effectiveness of such hydride descaling baths depends upon the readily oxidizable nature of the sodium hydride, the property responsible for the activity of the bath in reducing oxide scale to the metallic state. This same property, however, causes deterioration of the bath upon exposure to air since free oxygen, moisture and carbon dioxide may react with the hydride.

To avoid excessive loss of hydride, descaling baths have heretofore been provided with covers of various sorts preventing contact of the bath surface with the atmosphere. One type of cover consists simply of sheet steel pans floating on the surface of the bath. Such pans are not very satisfactory since they impede the immersion in the bath of articles to be descaled. In almost all operations where such a cover is used much of the surface must actually be left uncovered, the concomitant result being excessive decomposition of hydride.

Another type of cover which permits the active surface of the bath to be cut off from the air yet offers no obstacles to the insertion or removal of the workpiece is one which consists of finely divided particles dispersed over the molten surface. Heretofore graphite has been used in such dispersed covers. This material, however, is wetted by molten caustic and eventually sinks into the interior of the bath. Sinking of the cover not only leaves the bath open to surface oxidation but forms a sludge which may interfere with proper heat transfer therein. If the tank carrying the bath is heated at the bottom, heat insulation resulting from the sludge may cause the development of hot spots which may then burn through the tank. Removal of the sludge is a fairly diflicult operation and wastes both the time lost during the shutdown of the equipment and the caustic inevitably removed with the sludge. To avoid the formation of sludge the layer of graphite used as a cover has been kept very thin but under such conditions affords little protection to the hydride.

An object of this invention is, consequently, provision of a new and useful improvement in covers for fused salt baths. Another object is provision of a cover which will prevent air oxidation of fused salt baths containing metallic hydrides. Yet another object of the invention is to develop a protective cover for hydride descaling baths which will permit insertion and removal of the workpiece without 'difliculty but which will at the same time avoid the formation of an undesirable sludge. A further object of the invention is provision of a finely divided carbon cover for hydride baths which does not form carbon sludge therein.

The above-enumerated and still further objects of the invention may be accomplished by utilizing a thin layer of comminuted and sized charcoal as the covering on fused baths. Charcoal, unlike graphite, is evidently porous enough to entrap quantities of air internally. The entrapped air then imparts suflicient buoyancy to the charcoal to prevent the latter from sinking into the bath and forming sludge.

The charcoal of this invention is commercial charcoal comminutcd into particles sized to pass through sieves containing between about 8 and 20 meshes per inch. These figures correspond to particle diameters of 0.086 to 0.034 inch respectively. Ordinarily wood charcoals are in general quite satisfactory for the present usage.

The most important variable to be considered in making up the covers is the size of the individual particles. As

noted, the charcoal should preferably be rather coarse,

passing through 8 mesh but retained on 20 mesh sieves for best results. Such sized material will be referred to in this specification as 820 mesh charcoal. Particles smaller than 20 mesh, i. e., passing through 20 mesh sieves, tend to sink in the bath, particles smaller than mesh actually sinking so rapidly as to afford little advantage over graphite. Particles slightly larger than 8 mesh, i. e., up to about 6 mesh, can be used but with impaired results or in thicker covers. Particles in the preferred 8-20 mesh range both tend to float indefinitely and to give adequate protection in covers of acceptable thickness.

The thickness of the protective layer employed will depend to some extent upon the size of the particles constituting the layer. Particles of up to 8-20 mesh require a thickness of A to 2 in. for good results. In this preferred range the 8-20 mesh particles seem to give as good protection against oxidation as finer material does. This fact is quite surprising since the theory of protective powders suggests that extremely fine powder is less permeable to air. Covers of up to 6 in. thick may be successfully utilized, if desired, but are unnecessary. Graphite covers of such thicknesses cannot be used satisfactorily because the sludge formation. Since they must be rather thin, graphite covers give much less protection to the bath in the short time they remain afloat than the charcoal covers do. When present on a fused bath in the preferred thickness, that is, about A to 2 in., a charcoal layer will weigh about A to 2 lbs. for each square foot of area covered.

The cover of this invention may be formed by adding a small amount of comminutcd charcoal directly to the molten descaling baths. Preferably the layer should be built up of the desired thickness by sprinkling the powder uniformly over the surface of the bath. If desired, however, the charcoalcan actually be mixed into the bath. The coarser particles will then float to the surface and form the requisite protective layer. This second procedure is somewhat more involved than that first described and hence is less preferred. It may, in addition, lead to increased sludging.

The invention will be understood in more detail from a consideration of the following examples, in which all percentages are by weight unless otherwise noted:

Example 1 This example illustrates laboratory tests made to determine the effect of particle size on charcoal covers for hydride descaling baths.

The performance of a charcoal cover was observed for several weeks on a hydride bath having a surface area of two square feet. The bath consisted of molten sodium hydroxide containing l.3% by weight of sodium hydride. The cover was Ar /2 inch thick and was made up from cornminuted charcoal all particles of which passed through a 20 mesh sieve. Most particles were, however, retained on a 100 mesh sieve. The rate of decomposition of hydride within the bath was about 0.1% per hour before the cover was added. Decomposition of the hydride was almost completely stopped by the cover. A small amount of sludge gradually formed within the bath. It was found that the average particle size of the charcoal still floating gradually increased. Thus the bath preferentially wetted the smaller particles.

Example 2 This example illustrates the effects of stirring a charcoal cover.

A charcoal cover was made up as shown in the first example except that 61.4% of the particles were retained by sieves in the 6-20 mesh range. The rest of the charcoal passed through the 20 mesh sieves. The cover was stirred into the bath about twenty times per day for one week. At the end of this time a very small amount of sludge had formed and again the average particle size of the cover had increased.

Example 3 This example shows use of a charcoal-covered hydride descaling bath in a commercial establishment.

The graphite cover on an industrial hydride bath was replaced by powdered charcoal within the size range 8-2O mesh spread about A in. thick over the surface of the bath. The thin graphite cover previously utilized had afforded almost no protection to the bath but could not be made thicker because of the danger of excessive sludging. The charcoal cover reduced the consumption of the sodium used in generating the hydride to one-sixth its former value. The consumption of ammonia, cracked and reacted with sodium to form the hydride, was cut twothirds. The concentration of sodium hydride in the bath rose from 1% to 2% and the formation of caustic from decomposition of the hydride decreased until it was but slightly greater than the drag-out. Previously the removal and disposal of large amounts of caustic had been a problem. In addition to its other advantages, the charcoal cover insulated the bath and reduced the consumption of fuel.

The charcoal covers of this invention are, of course, beneficial to fused caustic baths other than those used for hydride descaling. Fused baths for case-hardening and heat treating are two types which readily come to mind. If powdered charcoal of the proper size, that is passing through approximately 820 mesh per inch, be chosen, the same advantages will accrue to these baths as to the descaling baths illustrated above. It will be obvious that charcoal should never be added to oxidizing baths, such as those containing nitrites or nitrates, with which it might react violently and dangerously.

Having described my invention, 1 claim:

In a process for descaling steel in a molten bath of sodium hydroxide containing about 120% by weight of sodium hydride, the step comprising supplying to said bath a cover of sized cornminuted charcoal, the particles of which are about 8-20 mesh in size, said cover being /4 to 2 in. thick.

References Cited in the file of this patent UNITED STATES PATENTS 2,377,876 Gilbert June 12, 1945 

